1. Field of Invention
The present invention relates to fuel burners and more particularly to an improved multi-fuel burner for reducing the formation of nitrogen oxides (NO.sub.x) by lowering the combustion zone temperature, decreasing the fuel and air mixing rate, and providing a reducing atmosphere in the combustion zone.
There is a present day growing concern with the immediate and long term problems created by the rapid increase in air pollution resulting from the rise in the industrial civilization level throughout the world. With this concerns comes an acute awareness that immediate steps must be taken to halt and reverse this upward trend in pollution. Great efforts are now being made by public and private economic sectors to develop measures for preventing potentially polluting particles and gases from being discharged into the atmosphere. One such source of atmospheric pollution is the NO.sub.x present in the stack emission of fossil fuel fired steam generating units.
Nitric oxide (NO) is an invisible, relatively harmless gas. However, as it passes through the vapor generator into the atmosphere and comes into contact with oxygen, it partially reacts to form nitrogen dioxide (NO.sub.2) or other oxides of nitrogen collectively referred to as nitrogen oxides (NO.sub.x). Nitrogen dioxide is a yellow-brown gas which, in sufficient concentrations, is toxic to animal and plant life. It is this gas which contributes to visible brownish haze in the atmosphere near industrial and metropolitan centers.
Nitrogen oxides are formed as a result of the reaction of nitrogen and oxygen at high temperatures and may be thermal nitric oxide and/or fuel nitric oxide. The former occurs from the reaction of the nitrogen and oxygen contained in the air supplied for the combustion of fossil fuel whereas the latter results from the reaction of the nitrogen contained in the fuel with the oxygen in the combustion air.
The rate at which thermal nitric oxide is formed is dependent upon any or a combination of the following variables: (1) flame temperature, (2) residence time of the combustion gases in the high temperature zone and (3) excess oxygen supply. The rate of formation of nitric oxide increases as flame temperature increases. However, the reaction takes time and a mixture of nitrogen and oxygen at a given temperature for a very short time may produce less nitric oxide than the same mixture at a lower temperature, but for a longer period of time. In vapor generators of the type discussed hereunder the combustion of fuel and air may generate flame temperatures in the order of 3,700.degree. F., the time-temperature relationship governing the reaction is such that at flame temperatures below 2,900.degree. F. no appreciable nitric oxide (NO) is produced via the thermal mechanism, whereas above 2,900.degree. F. the rate of reaction increases rapidly.
The rate at which fuel nitric oxide is formed is principally dependent on the oxygen supply in the combustion zone and the nitric oxide production is minimized under a reducing atmosphere; that is, a condition where the level of oxygen in the combustion zone is below that required for a complete burning of the fuel.
It is apparent from the foregoing discussion that the formation of thermal nitric oxide can be reduced by reducing flame temperatures in any degree and will be minimized with a flame temperature at or below 2,900.degree. F. and that the formation of fuel nitric oxide will be minimized by providing a reducing atmosphere in the ignition zone.
With the advent of stricter emission controls, manufacturers of fuel burning equipment have been actively seeking methods of limiting the amount of NO.sub.x pollutants which are formed from the combustion of fossil fuel. Heretofore, their efforts have been generally directed at either of the following two methods: two-stage combustion which calls for initial firing with a deficiency of air and the admission of the remaining air needed for complete combustion at a location remote from the burners, and another which calls for the addition of cooling surface in the combustion zone.